SERVER-15541 Clean up some usages of WriteUnitOfWork

Removes some WriteUnitOfWork objects where they are unneeded, and
moves the declaration of some existing WriteUnitOfWork objects to
limit their scope to the write being performed (so that
yield-capable reads are not inside them).
SERVER-14425: fix WriteConflictException handling in multi-update
SERVER-14860: fix multi-index build transactionality around setIndexIsMultikey
Merge remote-tracking branch 'nextdoor/master'